The present invention is generally related to the construction of semiconductor-laser modules and is more particularly related to the manner in which components of a semiconductor-laser module are affixed to one another so that resistance to fatigue is improved.
In some applications such as in laser printers and copiers, laser machining and in optical communication systems, lasers are operated intermittently in cycles with periods of several seconds or longer, perhaps several hours or even several days.
One type of laser module that is used in communications to pump rare-earth solid-state lasers includes an array or xe2x80x9cstackxe2x80x9d of laser subassemblies that are each affixed by some affixing agent to a substrate of electrical insulating material. In many embodiments, the laser subassemblies are soldered to the substrate, which in turn is soldered to a heat-dissipating component that stabilizes the operating temperature of the laser module. The heat-dissipating component may be a passive heat sink or it may be an active component such as a thermo-electric cooler or a liquid-coolant structure.
In one particular embodiment, each laser subassembly includes a rectangular prism or bar of semiconductor-diode laser material that is soldered lengthwise to a support member that provides the laser diode bar with structural support as well as an electrical contact to one side of the diode. The opposite side of the laser diode is coupled electrically to the support member of an adjacent laser subassembly in the stack by a plurality of wires along the length of the bar. These wires provide a plurality of parallel electrical connections between the opposite side of the laser diode bar and the support member of an adjacent laser subassembly. The laser diodes in the subassemblies of the stack are electrically coupled to one another in series.
Unfortunately, an unacceptably large number of laser modules that are operated intermittently stop operating after only a few thousand cycles or less. The laser modules sometimes experience a significant increase in operating temperature before they stop operating. In some cases, there is an opening of the electrical circuit that connects the laser diodes in the stack of laser subassemblies. In extreme cases, there is disintegration of the laser module into component subassemblies.
Several causes have been suspected and investigated. Empirical studies by the inventors have determined that the cause is a failure in the affixing agent used to affix components of the laser module to each another, particularly in a layer of solder used in many embodiments to affix laser subassemblies to the substrate and cooler. Furthermore, they have determined that this fatigue is a result of cyclical stress applied to the solder by unequal thermal expansion and contraction of the substrate and laser subassemblies as the laser module heats and cools from intermittent operation.
It is an object of the present invention to improve resistance to cycling fatigue in laser-diode stacks.
According to one aspect of the present invention, a method for designing a laser module comprises specifying a temperature range, specifying one or more materials from which two component of the laser module are to be made and specifying an affixing agent, specifying a structure in which the two component parts are affixed to each other by the affixing agent having a specified thickness, obtaining a measure of reliability for affixation of the two component parts as a function of the specified thickness, the specified structure, coefficients of thermal expansion of the one or more specified materials, and the specified temperature range, wherein the measure of reliability is obtained by estimating strain induced in the affixing agent by differences in thermal expansion of the two component parts across the temperature range, and adjusting the specified thickness of the affixing agent to obtain an acceptable measure of reliability. This method may be conveyed on a medium readable by a machine and embodying a program of instructions for execution by the machine to perform the method.
According to another aspect of the present invention, a laser module comprises a first component part made of a first material having a first coefficient of thermal expansion that is affixed by an affixing agent having a specified thickness to a second component part made of a second material having a second coefficient of thermal expansion, wherein the specified thickness of the affixing agent is established by a method that obtains a measure of reliability for affixation of the component parts as a function of the specified thickness of the affixing agent, the first and second coefficients of thermal expansion, and a specified temperature range, wherein the measure of reliability is obtained by estimating strain induced in the affixing agent by differences in thermal expansion of the component parts across the temperature range, and adjusts the specified thickness of the affixing agent to obtain an acceptable measure of reliability.
According to an aspect of the present invention, a laser module comprises a substrate of electrically insulating but thermally conducting material, and a laser subassembly comprising a bar of laser-semiconductor material that is affixed to the substrate by solder, wherein the solder has a thickness that is sufficiently great to obtain an affixation of the laser subassembly to the substrate with a measure of fatigue life exceeding about 80,000 cycles of strain caused by differences in thermal expansion of the substrate and the laser subassembly across a specified temperature range in which the laser module is designed to operate.
According to another aspect of the present invention, a method for manufacturing a laser module comprises forming a substrate of electrically insulating but thermally conducting material, forming a laser subassembly comprising a bar of laser-semiconductor material, and affixing the laser subassembly to the substrate with an affixing agent having a controlled thickness, wherein the method controls the thickness of the affixing agent to be substantially equal to or greater than a value derived from a design method that predicts reliability for affixation of the substrate and the laser subassembly as a function of the thickness.
The various features of the present invention and its preferred embodiments may be better understood by referring to the following discussion and the accompanying drawings in which like reference numerals refer to like elements in the several figures. The contents of the following discussion and the drawings are set forth as examples only and should not be understood to represent limitations upon the scope of the present invention.